It has heretofore been suggested to make microvoid-containing polymer particles to serve as hiding or opacifying agents in coating and molding compositions. Among the various procedures heretofore used, organic solvents and blowing agents occupy a prominent place therein.
Kreider U.S. Pat. No. 3,819,542, though containing no disclosure of the production of microvoid-containing polymer particles to serve as opacifying agents in coating compositions, is of background interest to show the use of organic solvents in a latex coating composition to produce a cellular film on drying. More specifically Kreider uses a mixture of a primary organic solvent, such as xylene, immiscible with the aqueous phase of the latex coating composition and a secondary organic solvent, such as propylene glycol, at least partially water-miscible and having lesser volatility and lesser solvating capacity for the polymer in the latex than the primary solvent. Upon evaporation of the primary solvent, a cellular film is obtained, the secondary solvent increasing the opacification.
Kurth et al. U.S. Pat. No. 3,875,099 discloses preparation of sequential acrylic polymers containing 0.5-2.5% of an alpha, beta-unsaturated carboxylic acid. The bulk of the acid is introduced in the early portion of the polymerization. Of the 11 examples, only Ex. 1 superficially resembles the film-forming thickener of the present invention. There is about 0.5% acid monomer over all, the first stage monomers containing about 13% methacrylic acid. The core/sheath weight ratio is in the neighborhood of 1/25. The core and sheath monomers are basically the same in this example (about 50:50 butyl acrylate methyl methacrylate), but the acid in the core confers a T.sub.i of about 30.degree.-35.degree. C., the sheath having a calculated T.sub.i of about 5.degree.-10.degree. C. Example 1 of Kurth et al. involves the use of a large amount of surfactant and a small amount of peroxy initiator, both of which are believed to normally result in extremely small latex particles. In our experience, this recipe would be expected to give core particles of about 0.04-0.05 micron average diameter, with the overall core-shell particle diameter being about 0.1-0.15 micron. The soft core, the small particle size, and the high temperature polymerization each may contribute to incomplete encapsulation. As shown by a comparative example herein below, the first stage of the Kurth et al. Ex. 1 is not adequately encapsulated.
Fantl et al, U.S. Pat. No. 3,401,134 discloses that cellulose ethers and water-soluble gums have been used to thicken aqueous coating compositions based on various polymer latices to thicken the composition for application, as by brushing, rolling, or the like. Fantl et al disclose that improved aqueous polymer dispersions are obtained by delaying the incorporation of acid monomer into the copolymer until at least 70% of the other monomers have been polymerized, asserting that the acid mer units are thereby oriented to the surface of the polymer particles and the acid copolymer thus obtained has the property of increasing in viscosity when the addition of a base, such as ammonia, raises the pH to 8 to 12.